The long-term objective of this proposal is to understand the genetic regulatory events underlying development of the mammalian embryo. This laboratory has begun characterization of a unique proto-oncogene family, the Ski/Sno family. Ski and Sno both have the paradoxical property that they can promote both oncogenic transformation and terminal skeletal muscle differentiation. Ski and Sno genes are highly conserved throughout evolution and closely related to each other, but are not related to other previously described protein families; they encode cell cycle-regulated nuclear proteins that are widely expressed and probably act as transcription factors. Preliminary evidence suggests that Sno functions in T lymphocyte activation, and Ski/Sno family function is essential to complete early development. We hypothesize that Sno and/or Ski function in embryonic development, affecting growth and development of multiple cell types, including hemo-lymphopoietic and neural lineages. The specific aims are to: 1) Characterize mice with a deletion of the 5' end of the Sno gene that have a T cell activation defect, to discover when Sno functions and with which other molecules it interacts. RAG-2-deficient blastocyst complementation will be used to study Ski function in lymphocytes for comparison. 2) Examine the function of the Sno gene using homologous recombination gene targeting in mice to make a null mutation, to detect functions in vivo that are Sno-specific and not also provided by Ski. 3) Examine the functional overlaps between Ski and Sno by intercrossing Ski/Sno double heterozygous mice and characterizing the embryonic lethal phenotypes in the double homozygous mutant in comparison with the single homozygous mutant and compound homozygous/heterozygous mutant embryos. The results will reveal which developing cell types require Ski/Sno family gene activity; gene dosage results will highlight differences between Ski and Sno functions. 4) Test the hypothesis that differences in phenotype between Ski and Sno knockout mice are due to differences in temporal expression rather than biochemical activities of the two proteins, using "knock-in" gene targeting to replace Ski coding sequences with Sno. These studies will establish the contribution of the Ski/Sno gene family to the genetic regulation of development in the embryo. Health-related benefits include possible insights into hemolymphopoietic cell function and birth defects.